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UC1548 Datasheet(Fiches technique) 6 Page - Texas Instruments

Numéro de pièce UC1548
Description  Primary Side PWM Controller
Télécharger  10 Pages
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Fabricant  TI1 [Texas Instruments]
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UC1548 Datasheet(HTML) 6 Page - Texas Instruments

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There are several methods to program IOFF. If accurate
maximum current control is required, IOFF must track out-
put voltage. The method shown in Figure 4 derives a
voltage proportional to VIN
? D (where D = duty cycle). In
a buck converter, output voltage is proportional to VIN
D. A resistively loaded diode connection to the bootstrap
winding yields a square wave whose amplitude is propor-
tional to VIN and is duty cycle modulated by the control
circuit. Averaging this waveform with a filter generates a
primary side replica of secondary regulated Vo. A single
pole filter is shown, but in practice a two or three pole filter
provides better transient response. Filtered voltage is con-
verted by ROFF to a current to the IOFF pin to control CI
If accurate system maximum current is not a critical re-
quirement, Figure 5 shows the simplest method of
downslope generation: a single resistor (ROFF = 40k)
from IOFF to VREF. The discharge current is then 100
The disadvantage to this approach is that the synthesizer
continues to generate a down slope when the switch is off
even during short circuit conditions. Actual inductor
downslope is closer to zero during a short circuit. The
penalty is that the average current is understated by an
amount approximately equal to the nominal inductor rip-
ple current. Output short circuit is therefore higher than
the designed maximum output current.
A third method of generating IOFF is to add a second
winding to the output inductor core (Figure 6). When the
power switch is off and inductor current flows in the free
wheeling diode, the voltage across the inductor is equal
to the output voltage plus the diode drop. This voltage is
then transformed by the second winding to the primary
side of the converter. The advantages to this approach
are its inherent accuracy and bandwidth. Winding the sec-
ond coil on the output inductor core while maintaining the
required isolation makes this a more costly solution. In the
example, ROFF = Vo / 100
A. The 4 ? ROFF resistor is
added to compensate the one volt input level of the IOFF
pin. Without this compensation, a minor current foldback
behavior will be observed.
Figure 4: Inductor Current Waveform Synthesizer
Figure 5: Fixed IOFF
Figure 6: Second Inductor Winding Generation of IOFF

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